For some reason my Gimp 2.10.18 is missing the ellipse selection tool...
You have tool groups enabled*. It's the default setting and a new feature in GIMP 2.10.18
The Ellipse Select tool is in the same group as the Rectangle Select tool. To find it, just click and hold down the Rectangle Select tool to see the Ellipse tool hidden under it, and select it.
You can tell which tools are groups by the tiny little arrow at the bottom right of each tool icon.
You can also select the tool using the keyboard shortcut E.
*Note: If you don't want to use tool groups, you can switch this feature off in Edit > Preferences > Interface > Toolbox - and uncheck the "Use tool groups" option.